Variable resistor

ABSTRACT

A resistive element has a gradient of resistivity extending along its length. It is held in slidable contact with two brushes spaced-apart from each other. The resistance between the brushes can be varied by changing their position relative to the resistive element and the gradient of resistivity. The resistive element may have an arcuate configuration and the relative position may be changed by rotating the element.

iiite States atent 1191 1111 3,760,322

Kogo et a1. [4 Sept. 18, 1973 VARIABLE RESISTOR 2,798,140 7/1957 Kohring338/137 [75] inventors:MasanoriKogo;HisashiHonda,both

OfTOkYOJaPa" 2,681,967 6/1954 Harrison et a1. 338/89 2,005,922 6/1935Stoekle 338/140 Assignee: Nippon Electric Companyjdiinited, I

Tokyo,Japan Filed: June 29, 1972 Appl. No.: 267,630

Foreign Application Priority Data June 30, 1971 Japan 46/48316 U.S. C1338/1137, 338/142, 338/150 Int. Cl 1101c 9/02 Field of Search 338/89,137, 138-142,

References Cited UNITED STATES PATENTS 10/1929 Greenewalt 338/141Primary ExaminerBernard A. Gilheany Assistant Examiner-D. A. ToneAttorneyNicho1 M. Sandoe et a1.

[57] ABSTRACT A resistive element has a gradient of resistivityextending along its length. it is held in slidable contact with twobrushes spaced-apart from each other. The resistance between the brushescan be varied by changing their position relative to the resistiveelement and the gradient of resistivity. The resistive element may havean arcuate configuration and the relative position may be changed byrotating the element.

5 Claims, 11 Drawing Figures PATENTEUSEHBIQB 3.760.322

' satmurd v Z (PR/OR ART) R HG. la Q FlGlb FIG. 2a 2b vanraatn nnstsronBACKGROUND OF THE lNVENTiON Tiis invention relates to a variableresistor for use in electrical circuits and, more specifically, to avariable resistor including a film of resistive material having agradient of resistivity in a predetermined direction.

in a conventional variable resistor, terminals are provided at the endsof the resistive film and a brush is held in slidable contact with thefilm to provide a variable resistance between one of the terminals andthe brush. The resistance then depends on the position of the brush withrespect to the film.

One of the shortcomings of this type of variable resistor is that theinductance component appearing in parallel with the variable resistanceundergoes a consider able change depending on the resistance selected. Aprincipal object of the present invention is therefore to provide avariable resistor capable of resistance variation without withoutcausing a change in inductance inductance component.

SUMMARY THE INVENTION THE INVENTION The variable resistor of the presentinvention comprises a resistive element having element having a gradientof resistivity that varys in a predetermined manner along its length andat least two brushes separated by a predetermined interval and held inslidable contact between the resistive desired The desired gradient ofresistivity can be given to the resistive element by tapering its widthor gradually changing its composition. More than two slidable brushesmay be used as required. When more more than two brushes are used, adifferent variable resistance can be across every two every two brushes.The brushes so interconnected so that the distance between them remainsconstant.

In the variable resistor of this invention, a desired variableresistance is provided between the slidable brushes, while maintainingthe accompanying inductance component at a fixed value.

Another feature of the present invention is that the variable resistancenever takes the value zero ohms. This contributes to the simplificationof a circuit including a finite minimum resistance.

BRIEF DESCRTPTTON OF THE DRAWTNGS The present invention will now bedescribed with reference to the accompanying drawings, in which:

FiGS. 1(a) and (b) show equivalent circuits ofa conventional variableresistor for the direct-current and extremely low frequency region andfor the high frequency region, respectively;-

FiGS. 2(a) and (b) show equivalent circuits of an embodiment of theinvention for the direct-current and low frequency region and for thehigh frequency region, respectively;

FlG. 3 is a cross-sectional view of a first embodiment of the invention;

FIG. 4 shows a circuit diagram of a bridged-T type variable resistanceattenuator to which this invention is applicable;

FIG. 5 shows a cross-sectional view of a second embodiment of of thisinvention adapted to the attenuator of HG. d;

FIG. ti; is an equivalent circuit ofa third embodiment of the invention;

FIG. '7 is an equivalent circuit of a fourth embodiment of theinvention;

FHG. b shows diagramatically, the impedance variation characteristics ofthe invention as compared to those of a conventional variable resistor;and

FIG. 9 shows diagramatically, the frequency characteristics of theconventional bridged-T type variable attenuator and those of theattenuator of FIG. 4 in comparison with the second embodiment of theinvention shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. ]l(a) shows an equivalentcircuit for a conventional variable resistor comprising a resistive filmit) having terminals l and 3 at its ends, and a slidable brush 2operating in the low frequency range. The desired variable resistance isobtained between the brush 2 and the terminal 3. The resistive film lit)has a uniform specific specific resistance, and its inductance componentis proportional to the length, but negligibly small when a directcurrent or extremely low frequency signal is applied. Likewise, thecapacitive component is also negligible. Generally, the impedance Z,between the terminals 1 and the brush 2 is given by:

where R stands for the resistance per unit length of the resistive film11411; L, for the inductance per unit length of the film x, for thelength of the section of the film iltl lying between the terminal llandthe brush 2; and w w, for the angular frequency. Differentiating Eq.(2) with respect to x and w,

From Eqs. (3) and (4), it is apparent that I Z,| increases linearly witheach successive increment of the length x, and non-linearly with eachsuccessive increment of the angular frequency w.

The impedance Z, of an ideal variable resistor having neither acapacitance component nor an inductance component may be expressed asfollows:

where resistive film is assumed to have a uniform specific resistance R.The ratio A, of |Z l to |Z,| is given More specifically, the absolutevalue of impedance is always larger in the conventional variableresistor than in the ideal variable resistor. Also, the larger theangular frequency w, or the smaller the specific resistance R of theresistive film, the greater the ratio A, of Eq. (6) becomes. in otherwords, with the increase of the frequency, the impedance has a greatereffect on the variable resistance, making the deviation from the idealvariable resistor greater. An equivalent circuit for a conventionalvariable resistor at high frequency is shown in MG. i(b).

in the equivalent circuits of the variable resistor of the the inventionshown in FlGS. 2(a) and slidable slidable brushes 5 and 6 are in contactwith aresistive film 30. The film 30 (resistive element) has a nonlineargradient of resistivity respect respect to its length. Two brushes 5 and6, spaced by a predetermined a predetermined interval, are held contactslidable contact with the resistive film 30. between interval betweenthe brushes and the nonlinear gradient in the resistivity of the film 30are that the so that the resistance and the brushes 5 and 6 exhibits anproportion in linear proportion movement the lengthwise movement of willbe brushes. It will be apparent, is not that the invention is notlimited to this resistive film arrangement. In general, the gradient maychange continuously, linearly or nonlinearly. For simplicity in thisexplanation, it is assumed here gradient, in other in the gradient, inother words, the rate of the change of the resistivity of linear film30, is in linear proportion its length. the position selected along itslength.

Assuming that the distance from the lefthand end of the resistive film30 to the brush 6 is x, the resistance therebetween is xR, and thespatial interval between the brushes 5 and 6 is unity, then theimpedance Z earse. hebwsh 5 and 6 an be aasam.

where x a l and the variables are the same as those in Eqs. (1) and (2).The ratio A of the impedance lZ i of the present variable resistor tothe impedance iZ l of the ideal variable resistor is given by:

9) where x Derived from Eqs. (2) and (3), the ratio A of the air--solute value of the impedance of the present variable resistor to thatof of the conventional one is given b In other words, in the range of x2 1.0, the variable impedance Z of the present variable resistor issmaller than that Z of the conventional resistor and closer to that ofthe ideal resistor. FIG. 8 shows the value of A given in Eq. (10) withrespect to the length x.

In such a variable resistor, the resistance obtained across the brushes5 and 6 cannot take the value zero ohms even under the state of unitythus maintaining a certain minimum resistance value. The variableresistor of this invention is therefore highly desirable for thosecircuits which require a minimum constant resistance.

Referring now to H6. 3, the first embodiment of the invention, anequivalent circuit of which is shown in FIG. 2, two brushes 106 and 107are attached to a cylindrical housing 105 and held in contact with anarcuate resistive film 102 attached to a disc-shaped base I03. The film102 may form a complete circle. The resistive film 1102 has a nonlineargradient of resistivity in the circumferential direction. This gradientis attained by tapering the thickness or the width of the film 102.Alternatively, it may be attained by varying the composition of thematerial of which the film is made. By the expression gradient ofresistivity it is meant that the resistance per unit length of the filmvaries in a predetermined manner as as it is measured at differentpoints along its length. The base 103 is pressed downwardly by a spring104% thus permitting rotation under the control of a rotatable disc1101. The desired variable resistance value is thus obtained between thebrushes I06 and 107, by changing the relative position of the brushesand the film 102.

Referring now to FIG. 4, a bridged-T type variable attenuator to whichthe present invention is applied permits a variation in the attentuationdepending on the resistance of resistors Rs and Rp which is selected.When conventional variable resistors are used in this attenuator, thevoltage standing wave ratio (VSWR) is unavoidably increased with anincrease in the frequency. This tends to cause an increase in theimpedance Z Referring now to FIG. 5, a second embodiment is shown whichis adapted to the variable attenuator of FIG. 4 and has a dual variableresistor which may be viewed as two separate variable resistors. Each ofthese variable resistors is the equivalent to one resistor of thecircuit shown in FIG. 2. The dual variable resistor has two brushes 11Mand 117 fixed to a base 119 which also serves as a cover. Two arcuateresistive films 112 and 112 having nonlinear resistivity gradients areattached to bases I13 and 1M and kept in slidable contact with thebrushes 116 and 117. The resistive films 112 and 112' are moved as adisc 101i on which the bases I13 and 1114 are mounted is rotated. Theseelements are contained in a cylindrical housing 105 and a closureincluding a base 119. A pair of brushes are provided for each of theresistive films 112 and T12.

With the dual variable resistor of FIG. 5 used as the resistors Rs andRp of the the bridged-T variable attenuator variable attenuator of FIG.4, the impedances of the resistors Rs and Rp exhibit virtually no changein the inductive components, as indicated by Eq. (7). Therefore, thevoltage standing wave ratio (VSWR) can be kept unchanged as shown inFIG. 9, which illustrates two groups of curves showing the attenuationvs. frequency and the voltage standing wave ratio (VSWR) vs. frequencycharacteristics of the present variable attenuator as those with thoseof variable-resistor-based attenuator. variable-resistor-basedattenuator.

In the bridged-T type type variable attenuator of FIGA, resistancesresistances r and r of the resistors Rs and Rp can be expressed as afunction of value attenuation value R0 as follows:

r R0(e l)i Where a is given in neper.

The zero attenuation value is achieved by making the resistance rs andrp zero and infinity, respectively. Conversely, infinite attenuationachieved achieved by making rs and rp infinity and zero, respectively.Practically, however, these requirements cannot be met, infiniteimpedance impdance is difficult to achieve with a conventional variableresistor. This results in a residual attenuation which causes a highvoltage standing wave ratio. Therefore, to achieve a satisfactorily lowvoltage standing wave ratio with the conventional device, at least oneresistor must be in series with either its or Rp or both. In contrast,the variable resistor of the invention never takes the value zero ohms.This makes it it possible to dispense with any additional resistance elements and consequently to simplify miniatuarize miniaturarize theattenuator as a whole. This applies to any apparatus or device in whichthe present invention finds application.

Referring now to FIG. 6, a third embodiment of this invention has fourbrushes 202, 203, 204, and 205 spaced at predetermined intervals andheld in slidable contact with a resistive film 201. This variableresistor makes it possible to to obtain two desired variable resistancevalues from one resistive film 201, one from the brushes 202 and 203 andthe other from the brushes 204 and 205. In other words, a dual variableresistor can be constructed using one resistive film 201i. Brushes 203and 204 may be connected to each other when these two brushes are to beat an equal potential. It will be apparent that more than four brushescan be used.

Referring now to FIG. 7, the fourth embodiment of the invention includesa resistive film 30I having terminals 304 and 305 at the ends,respectively, and brushes 302 and 303 held in slidable contact with theresistive film 3011. With a voltage applied between the terminals 304and 305, the desired voltage is derived from the brushes 302 and 303. Inthis variable resistor, the inductive component of the output impedanceimpedance is small and constant. Therefore, the use of this variableresistor in a high frequency device will contribute substantially tobetter performance.

In the foregoing description, we have explained the present inventionwith reference to several exemplary embodiments and their application.However it will be apparent to those skilled in the art that othermodifications are possible without departing from the spirit and scopeof the invention.

We claim:

I. A variable resistor comprising an electrically resistive element theresistance of which changes along its length in a predetermined mannerto form a gradient of resistivity, and at least two spaced-apart movablebrushes connected to each other so that the distance between them isfixed and in slidable contact with the resistive element, whereby theresistance obtained between the brushes can be selected bypositioningthe brushes relative to the resistive element and the gradient ofresistivity but the inductance between the brushes remains substantiallyconstant regardless of the position selected.

2. A variable resistor set forth in claim I, wherein said brushes andsaid resistive element are supported by a housing whereby said brushesare held against said resistive element.

3. The variable resistor set forth in claim ll, further comprising adisc-shaped member to which said resistive element is attached.

0. The variable resistor set forth in claim 3, further comprising ahousing by which said disc-shaped member is rotatably supported.

5. The variable resistor of claim I, further comprising a cylindricalhousing and a disc-shaped member rotatably supported within the housing,the resistive element having an arcuate configuration and being attachedto the disc-shaped member for rotation therewith.

Patent No.

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Page 1 of A Inventor(s)KOqO, M. et a1 It is certified that error appearsin the above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 1 4, Iiis" should be --This--;

delete "without" (1st occurrence) delete "inductance";

delete "element having" (lst occurrence) delete "every two";

Column 2 delete "specific" (lst occurrence);

"andthe" should be --and the--;

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delete w delete "the" (lst occurrence) after insert -2(b)--; delete"slidable";

"so" should be -are-;

II II o w po-m lo MCI-(-29) Page 2 01" UNEEED STA'lES PATENT ()FFICE(IER'H FEE/VIE (W CQRR M11310 N Patent No. ,7 1 Dntmh QQBEQQQQPInventor(s) Kogo, M. et al It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 3 Line 2, change "respect"(lst occurrence) to with-; Line 3,after "spaced" insert ---apart--; delete "a predeterminedflstoccurrence); change "contact" to ---in--;

Line 5, after "30," change "between" to --The-;

Line 7, change "that the" to -selected-;

change "and" to -between--;

Line 8, change "proportion" to --increase;

change "movement" to --to--;

Line 9, change "will be" to the;

Line 10, change "is not" to -however-;

change "gradient in other" to -that the change-;

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Line 16, change "its length" to -to-;

Line 32, Z 2,, should be 52 g 52 E lage 5 of UNE'EED STATES PA'EESNTOFFICE GERTENQATE UP CGRREUHN Patent No. 3,760 ,322 Dated September 18,1973 Inventor(s) K090, M. et al It is certified that error appears inthe above-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 4 Line 17, "attentnation" should be attenuation;

Line 42, delete "variable attenuator";

Line 50, after "as" change "those" to compared--;

after "of" insert a conventional--; Line 51, after "attenuator"(lstoccurrence) delete "variable resistor based attenuator.-;

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Line 54, after "of" change "value" to the- Line 61, after "attenuation"change "achieved" to ---is--;

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-' OHM 0-1050 (1O 19) Page LP of UNHED STATES PATENT OFFICE CERTlFlQATEUP CURRECTIQN Patent No. 3,760 ,322 Dated September 18, 1973Inventor(s)KOg'O, M. et al It is certified that error appears in theabove-identified patent and that said Letters Patent are herebycorrected as shown below:

Column 5 Line 4, after "it" delete "it"; Line 6, change "miniaturi'ze"t0and;

Line 14, after "to" delete -to-;

Line 30, after "output" delete "impedance f' Signed and sealed this 12thday of November 1974.

(SEAL) Attest:

MCCOY M. GIBSON JR c. MARSHALL DANN Attesting Officer Commissioner ofPatents FORM F'O-lOfll) (HLFIH

1. A variable resistor comprising an electrically resistive element theresistance of which changes along its length in a predetermined mannerto form a gradient of resistivity, and at least two spaced-apart movablebrushes connected to each other so that the distance between them isfixed and in slidable contact with the resistive element, whereby theresistance obtained between the brushes can be selected by positioningthe brushes relative to the resistive element and the gradient ofresistivity but the inductance between the brushes remains substantiallyconstant regardless of the position selected.
 2. A variable resistor setforth in claim 1, wherein said brushes and said resistive element aresupported by a housing whereby said brushes are held against saidresistive element.
 3. The variable resistor set forth in claim 1,further comprising a disc-shaped member to which said resistive elementis attached.
 4. The variable resistor set forth in claim 3, furthercomprising a housing by which said disc-shaped member is rotatablysupported.
 5. The variable resistor of claim 1, further comprising acylindrical housing and a disc-shaped member rotatably supported withinthe housing, the resistive element having an arcuate configuration andbeing attached to the disc-shaped member for rotation therewith.